title: The heritability of clinically diagnosed attention deficit hyperactivity disorder across the lifespan.
abstract: BACKGROUND
  No prior twin study has explored the heritability of clinically diagnosed attention deficit hyperactivity disorder (ADHD). Such studies are needed to resolve conflicting results regarding the importance of genetic effects for ADHD in adults. We aimed to estimate the relative contribution of genetic and environmental influences for clinically diagnosed ADHD across the lifespan with a specific focus on ADHD in adults.
  
  
  METHODS
  Information on zygosity and sex was obtained from 59514 twins born between 1959 and 2001 included in the nationwide population-based Swedish Twin Registry. Clinical data for ADHD diagnoses (i.e. stimulant or non-stimulant medication for ADHD) were obtained from the Swedish Prescribed Drug Register (PDR) and from the National Patient Register (i.e. ICD-10 diagnosis of ADHD). Twin methods were applied to clinical data of ADHD diagnoses using structural equation modeling with monozygotic (MZ) and dizygotic (DZ) twins.
  
  
  RESULTS
  The best-fitting model revealed a high heritability of ADHD [0.88, 95% confidence interval (CI) 0.83-0.92] for the entire sample. However, shared environmental effects were non-significant and of minimal importance. The heritability of ADHD in adults was also substantial (0.72, 95% CI 0.56-0.84).
  
  
  CONCLUSIONS
  This study shows that the heritability of clinically diagnosed ADHD is high across the lifespan. Our finding of high heritability for clinically diagnosed ADHD in adults indicates that the previous reports of low heritability are best explained by rater effects, and that gene-identification studies of ADHD in adults need to consider pervasiveness (e.g. multiple raters) and developmentally (e.g. childhood-onset criteria) informative data.
authors.affiliation: Department of Medical Epidemiology and Biostatistics, Karolinska Institutet, Stockholm,Sweden.
authors.forename: H
authors.initials: H
authors.lastname: Larsson
authors.affiliation: Department of Medical Epidemiology and Biostatistics, Karolinska Institutet, Stockholm,Sweden.
authors.forename: Z
authors.initials: Z
authors.lastname: Chang
authors.affiliation: Department of Psychological and Brain Sciences,Indiana University,Bloomington, IN,USA.
authors.forename: B M
authors.initials: BM
authors.lastname: D'Onofrio
authors.affiliation: Department of Medical Epidemiology and Biostatistics, Karolinska Institutet, Stockholm,Sweden.
authors.forename: P
authors.initials: P
authors.lastname: Lichtenstein
mesh_terms: D000293:Adolescent; D000328:Adult; D001289:Attention Deficit Disorder with Hyperactivity / Q000453:epidemiology / Q000235:genetics*; D004200:Diseases in Twins / Q000453:epidemiology / Q000235:genetics*; D005260:Female; D020022:Genetic Predisposition to Disease; D006801:Humans; D008297:Male; D012042:Registries; D013548:Sweden / Q000453:epidemiology; D055815:Young Adult